Origin of Dry Chalk Valleys and of Coombe -Rock. 137 



the spot where the famous Pink and White Terraces existed. Before 

 the recent eruption there were do craters on Mount Tarawera, the 

 form of which was a ridge, apparently due to denudation. 



A description of the eruption was then given. Although earth- 

 quakes had prevailed for some time in the neighbourhood, they were 

 not violent. Shortly after midnight on the 10th June a series of 

 tremendous explosions took place from various parts of the Tarawera 

 ridge, and columns of steam were thrown up with quantities of red- 

 hot stones. The whole mountain appeared as if on fire. A column 

 of steam was then sent up from near Okaro far to the west, and, 

 finally, a great explosion took place in Lake Eotomahana, and 

 steam rushed forth to a height exceeding that of the columns from 

 Tarawera. These eruptions from the plain were not accompanied 

 by any red-hot stones : the ejecta were of much lower temperature. 

 The principal eruption, accompanied by violent earthquakes and 

 loud noises of various kinds, was over by 5.30 a. m., and the moun- 

 tain craters ceased to be active within twenty-four hours, but steam 

 with some stones and mud continued to issue from the Eotoma- 

 hana and Okaro craters for several days, and steam has ever since 

 been emitted from Eotomahana. 



The results of the eruption in the form of fissures on Mount 

 Tarawera, the change of Eotomahana from a lake to a crater of 

 larger dimensions, with precipitous walls, the formation of a new 

 lake between this crater and Tarawera, and the formation of a 

 number of small craters about Okaro, were then briefly noticed. 

 The materials ejected were composed of augite-andesite, and 

 rhyolites, both compact and vesicular. The mineral structure and 

 distribution over the surrounding country of various forms of 

 pumice, scoria, and ash were described, and it was shown that 

 there was a difference in the substauces ejected from the moun- 

 tain craters of Tarawera and those from the craters in the plain 

 at Eotomahana and Okaro, the former comprising pumice and 

 scoria, which were not thrown out from the latter, and but little 

 steam issuing from the higher craters when compared with the 

 enormous volumes emitted from the lower vents. 



The cause of the eruption was ascribed to the reheating of old 

 lava-streams saturated with water. This reheating was appa- 

 rently not due to crushing; for, had it been so, the preceding earth- 

 quakes would have been more violent ; but probably to molten rock 

 coming up from below and heating the rocks near the surface. The 

 eruptions from Eotomahana and Okaro were purely hydrothermal. 



February 23.— Prof. J. W. Judd, F.E.S., President, in the Chair. 



The following communication was read : — 



1. "On the Origin of Drv Chalk Yalleys and of Coombe Eock.'' 

 By Clement Eeid, Esq., F.G.S. 



Whilst engaged in examining the Pleistocene deposits of Sussex, 

 for the Geological Survey, the Author observed that the Coombe 

 Eock differs from anything commonly seen in the strongly glaciated 

 districts of the Yorkshire and Lincolnshire Wolds. As in these 

 localities, the seaward slope of the South Downs is broken by the 



